Conventionally, in communication systems where packet data is transmitted to a plurality of communicating stations, scheduling techniques and adaptive modulation techniques are introduced.
The adaptive modulation technique is a technique for varying the modulation level of M-ary modulation and coding rate corresponding to the state of a communication path in transmitting packet data, and thereby transmitting data as fast as possible while achieving a desired error rate. Specifically, the modulation level and coding rate are both increased to a communicating station with a good state of the communication path, and thereby data is transmitted at a high rate. Meanwhile, the modulation level and coding rate are both decreased to a communicating station with a poor state of the communication path, and thereby a desired error rate is satisfied.
The scheduling technique is often used in a combination with the adaptive modulation technique. A scheduler permits transmission of packet data preferentially to a communicating station with a good state of the communication path, thereby enhancing the throughput of the entire system. As a representative scheduler, there is “MAX C/I scheduler” that transmits packet data preferentially to a communicating station with a large CIR (Carrier to Interference Ratio) indicative of quality of the communication path.
As described above, in a communication system using “MAX C/I scheduler”, when the state of the communication path deteriorates, a transmitting-side apparatus does not permit transmission to a communicating station with a poor state of the communication path. Therefore data cannot be transmitted. Also, in a system applying the adaptive modulation technique, when the state of the communication path becomes too poor, the desired error rate is not satisfied if the modulation level and the coding rate are both decreased. In such a case, data cannot be transmitted.
Herein, a case is considered of transmitting successive packet data (for example, streaming data). On the receiving side, since data cannot be received, processing capabilities of processor and required capacity of buffer memory for reception are reserved until data can be received. Further, in a lower layer of the receiver, since data is provided to an upper layer only after data is received to some extent, data is not provided to an upper application unless data reception is completed to some extent, and thus, the data is not provided to the upper layer soon.
Therefore, in a receiving station whose state of the communication path becomes poor, a reception buffer is reserved and processing of processor is reserved although data is not transmitted, and there arises a problem of generating wastes in memory and processing capability.
Further, since the lower layer does not provide data to the upper layer, in a moving picture application, the entire screen is not displayed, despite even only part of the screen being displayed if the lower layer provides part of the data.
Furthermore, in a structure where the streaming data is connected via a network (such as the internet) to an apparatus ahead of the transmitter, the upper layer cannot judge whether a problem occurs in a server and network that transmits data or state of the communication path between the transmitter and receiver deteriorates, and therefore, performs retransmission control on the server connected to the network. In other words, when only the state of the communication path deteriorates, a retransmission request needs to be transmitted to only a radio transmission apparatus without being transmitted to the server. However, since why the data is not transmitted cannot be judged, a retransmission request is made to the server connected to ahead of the network, and there arise problems that loads on the server are increased and that traffic to the network is increased.